In a new concept for a long-term evolution (LTE) of 3GPP (3rd Generation Partnership Project), the assumption is that multiple users can be multiplexed in the downlink direction within a single sub-frame, which may have a time duration of 1 ms, or within several consecutive sub-frames. Currently, the working assumption is that a transmission timing interval (TTI) for scheduled transmissions may have a duration of 1 ms. However, there are also discussions on whether it should be possible to have a shorter duration of a single user allocation when having persistent allocations. Current 3GPP terminology states that a 0.5 ms entity is a slot, while a pair of two slots constitutes a subframe, which in turn is the same length as the TTI. One of the expected key features for the LTE is extensive usage of HARQ (hybrid automatic request process) to enable fast recovery of erroneously received data packets. For each transmitted packet, physical resources will be allocated in the uplink so that each allocated user equipment (UE) can transmit ACK/NACK (acknowledgement) information based on its reception. The assumption for the downlink is that the HARQ is asynchronous, but it is expected that the UE's transmission of the ACK/NACK information (can be also referred to as ACK/NACK) will be time-wise tied to the associated downlink transmission. In cases where the UE does not have data to transmit in the uplink at the time of the ACK/NACK (data-non-associated control signaling) transmission, a dedicated physical control channel (e.g., similar to a high speed dedicated physical control channel, HS-DPCCH, for high speed downlink packet access, HSDPA) is assumed to carry the acknowledgement information (e.g., an ACK/NACK bit). Otherwise, in case of data-associated control signaling, the ACK/NACK information can be piggy-bagged to the data transmission. Both the allocation in uplink and downlink will be decided by the Node B.
A number of users multiplexed in downlink may change significantly from one sub-frame to another. Some of the factors contributing to such variations include:    changes in traffic (burstiness) which means that varying number of users have different and fast varying amounts of data to transmit;    issues related to different TTI sizes in uplink and issues of whether TTI size is cell or user specific and if it is paired to the downlink transmission or not;    properties of radio-aware scheduling that cause variations in the number of users allocated per the sub-frame, etc.
For an asymmetrical traffic (or time-shifted as in the case of voice over internet protocol, VoIP), it will often happen that the ACK/NACK needs to be sent in uplink as “data-non-associated” transmission, e.g., on a separate physical channel tied to the downlink allocation. One option would be to reserve “space” for all active UEs, such that they all have a reservation channel for signaling corresponding to ACK/NACK. Since the uplink resources are orthogonal in time/frequency, any unused transmissions (even for control signaling) will imply that system capacity/throughput are wasted whenever such situations occur. In order to reduce a the HARQ related signaling overhead/resource waste, resource assignments for such control signaling messages can be scheduled, but this would lead to excessive overhead in the downlink control signaling.
For the HSDPA, an associated control channel was set up for each data connection (the HS-DPCCH). However, for the LTE, it is assumed that the uplink transmissions will use a single-carrier FDMA (frequency-division multiple access), where the channel resources are inherently orthogonal to the different users, unlike CDMA (code-division multiple access) where all active UEs are transmitting at the same time and frequency, so a pre-reserved control channel in the uplink for acknowledging the downlink data would lead to a loss of uplink capacity if utilization is not 100%.